"D/B" on shifter?

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Yanquetino

Well-known member
Joined
May 11, 2010
Messages
479
I am intrigued by this photo of the Leaf's shifter:

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It clearly shows a mode labeled "D/B". The "D" obviously stands for "Drive." As for the "B", I am guessing it might stand for "Braking," i.e., the ability to "downshift" when descending steep hills. At least this is what "B" stands for on a Prius, as well as on the Mitsubishi iMiEV:

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I understand that in the iMiEV, its "B" mode increases the regenerative braking, so I am guessing that would likely be the case in the Leaf as well.

Am I guessing correctly? If so, how does one select "B"? By pulling the knob down a second time? And then how does one resume normal "D" mode? By pulling it down yet again? In other words, do the "D" and "B" modes toggle back-and-forth between "normal" and "strong" regenerative braking?

Also, when the "B" mode is engaged, do drivers see this on the dashboard display? (I have only seen "N" for "Neutral" in photos, but assume that the letter must change when shifting, right?)

I hope that I am correct in my interpretation of "B", as I believe that U.S. regulations mandate that a car must have a way of "downshifting" for steep declines --lest drivers "ride" their brakes.

Thanks!
 
Good question.

Perhaps in the D/B mode one has both Driving and regen Braking, variable Driving with the "gas" pedal, and variable Braking (regen) with the first part of the brake pedal?

And, to be legal it must have a mode labeled "B" (auto-B mode, invoked when pressing the brake pedal), which also happens to be the "auto-D" mode (touching the accelerator)?
 
Yanquetino said:
I am intrigued by this photo of the Leaf's shifter:
I understand that in the iMiEV, its "B" mode increases the regenerative braking, so I am guessing that would likely be the case in the Leaf as well.
That has been the assumption, and it is a reasonable one. As to how it is engaged, though, I have not seen anything on that yet. I have tried to look at photos of the dash and of this section as well looking for an additional switch that would engage the "brake" mode, and have so far failed to discover the activation method.

For now, your toggle theory is the best one I've seen. Since there is only one indicator light below the shifter, I would imagine that the main dash would indicate the change between D and B mode.
 
Since there is no real "transmission" in the Leaf, why would anyone assume there is a difference between "Drive" and "Brake" (Engine Brake)? If you take your foot off the acelerator, you are automaticlly engaging the engine (motor in this case) "brake" because you are not putting energy into the motor... There will be drag on the motor, and it will slow down and do some regen, and why they show it as just 1 position. No distinction needs to be made, if you are not acelerating or staying the same speed (foot at the same position on the accelrator), you are "braking" (B). This is without touching the actual brake pedal...
 
mitch672 said:
Since there is no real "transmission" in the Leaf, why would anyone assume there is a difference between "Drive" and "Brake" (Engine Brake)?
If this works like some of the other EV vehicles work, it would change the software to a more aggressive engine braking profile. This can be useful in hilly areas, or even just someone that wants to take more advantage of braking regen.
 
The B-mode, if it exists as a distinct mode, will undoubtedly increase the regen (motor) braking, to aid going down hills.

My Prius does that, but even the increased regen was not very "strong", not at all like downshifting a typical ICE vehicle. Perhaps more like not downshifting an ICE vehicle. On a steeper hill (behind Laguna Beach), I had to use the Prius' mechanical brakes "all the way" down.
 
There is a very important distinction between a pure EV and a hybrid, such as the Prius: Both can use regenerative braking to slow the car, and both can use a "B" mode to increase that braking. The difference comes when the battery is full and there is no place to send the electricity. The Prius still has an internal combustion engine and it can (and does!) use engine compression braking when the battery is full and "B" mode is engaged.

Contrary to what Mitch states above, there is no automatic drag on the motor when you lift your foot off the accelerator. An electric motor is freewheeling when no contacts are applied. It can be configured (by the car's computer) to act as a generator, and that applies drag, but that also produces electricity, and that electricity must go somewhere! As long as the battery is not full, it can go into the battery and all is well. We are even recovering energy. But once the battery is full we have a serious problem: Energy does not disappear! Engine compression braking turns that energy into heat (by compressing air in the cylinders) and dumps that heat through the exhaust system. But an EV does not have an engine.

A pure EV must have some way to dump the energy of slowing the car if the battery is full. The situation will be rare, because the large battery will usually have room to absorb the energy of regen braking under normal conditions. But if you live on top of a hill and you fully charge the battery and then start down a long, steep hill, you run the risk of overheating the friction brakes. (Under normal circumstances, a conventional car converts kinetic energy into heat in the brake linings, and that heat is dissipated in the time between braking events.)

The problem could be solved by dumping excess electricity (from regen braking after the battery is full) into the resistive heater, and then venting excess unwanted heat to the outside.

My question is, does the Leaf do this, or does it rely only on friction braking for long descents after the battery is full???

I'd be very interested in an answer from Nissan on this.
 
I'd like to see some calculations - but my guess is that even if you practically start on top of a hill - a BEV battery will have space for regen. Most BEVs will not top off the battery & thus will have some space left.
 
The brakes on the Leaf will be larger than required so braking with a full pack will be adequate. When an EV is fully charged the regen can be significantly reduced as it is directly related to the load on the motor and most will not allow higher currents to a fully charged pack but more importantly the pack will not supply the high load as it is full, this happened often on my EV as I live at the top of a hill and the regen was extremely low when fully charged, it was even software restricted below the load of the pack for protection form overcharge. There is a way to shunt power to a resistive load for regen loading or in those cases where the pack is fully charged the regen will be significantly reduced or cut off. The best option is to move to the bottom of a hill but I'm sure this will not be an issue for the Leaf:)
 
numbers such as discharge down to %10 SOC and charge to %90 SOC have been bandied about (using %80 of the usable capacity of the battery pack), the very rare condition of the battery pack being at %90 SOC at the start of a long descent would be very rare indeed, unless you live at the top of a large mountain or hill, that would be the only time it could even be an issue. I'd bet that Nissan just reverts to friction braking in those rare conditions, you can't count on running the resistive heater, it could be the middle of summer, and unless they made a way to dump that heat outside, it would could get very hot in the passenger compartment. Of course you could run the air conditioner and turn on the lights at that point, so I guess there is always something they could do, such as run the AC/Lights in the summer/warmer weather, and run the resistive heater in the winter/colder weather... I wonder if they have even considered this as a possible condition, being how truly rare it would be? I suppose they could also have a special "resistive load" under the hood, that would be air (or liquid) cooled via the front vent, it would only be used in this special situation, kind of a "load bank" for excess energy.
 
Bottom line, they have engineered some solution. My guess is the cigarette lighter glows bright under these conditions.
 
Regardless of what maximum percent SoC they allow the battery to reach, once it reaches that, they either overcharge the battery, revert to 100% friction breaking, or use some other method to dump energy. And regardless of how rare the situation of a full battery at the top of a long hill is, if it can happen, then they have to have some way to prevent overheating of the friction brakes, because the alternative could be disastrous.

All I'm asking is for Nissan to tell us if they've thought of this (I presume they have, but I'd like to hear them say it) and what particular method(s) they've implemented to deal with it. There are no technical difficulties involved, as there are various ways to do it. I'd just like to know, because it's not a trivial issue. Conventional cars and hybrids have a backup in the form of compression braking. I'd like to know what the backup is for the Leaf. Not speculation about what might be, but the straight word from Nissan about what is.
 
Don't diesel-electric locomotives use giant resistors with fans on downgrades?

For the Leaf you should make it down safely from a big mountain pass on the friction brakes if they are sized appropriately, although I never like to ride the brakes down a hill even in a car let alone a truck. As pointed out this seems like it would only be an issue if your house was on the top of the hill, but there are certainly cases where that is true.

Maybe we'll start to see signs that say "EV's Welcome!" on runaway truck ramps.

If they are relying just on friction brakes for this scenario, that raises an interesting question... sized for this worst case scenario that would never happen for most drivers, combined with the regen those brakes will probably last the life of the car.
 
daniel said:
All I'm asking is for Nissan to tell us if they've thought of this (I presume they have, but I'd like to hear them say it) and what particular method(s) they've implemented to deal with it.

Daniel - It's good that you care about having brakes that work. But let me start with a question: What do you drive now? Do you have a statement from the manufacturer that they sized the brakes correctly for the car? Actually, you do. If the car is certified to be on US roads, it must meet US safety standards - and that includes brake performance.

"These Federal safety standards are regulations written in terms of minimum safety performance requirements for motor vehicles or items of motor vehicle equipment. These requirements are specified in such a manner "that the public is protected against unreasonable risk of crashes occurring as a result of the design, construction, or performance of motor vehicles and is also protected against unreasonable risk of death or injury in the event crashes do occur."

Standard No. 135 - Light Vehicle Brake Systems - Passenger Cars (Effective 3-6-95), Multipurpose Passenger Vehicles, Trucks and Buses (Effective 12-1-97)

This standard specifies requirements for vehicles equipped with hydraulic and electric service brakes and parking brake systems to ensure safe braking performance under normal conditions and emergency conditions. Manufacturers of passenger cars and multipurpose passenger vehicles, trucks and buses with a gross vehicle weight rating less than or equal to 3,500 kg (7,716 lbs.) may certify compliance with either FMVSS No. 105, described earlier in this booklet, or FMVSS No. 135. The options expire on September 1, 2000 for passenger cars and on September 1, 2002 for other vehicles, on which dates compliance with FMVSS No. 135 is mandatory.


How will this affect you? The FMVSS standards are designed to assure new vehicles are capable of stopping within a certain distance deemed necessary for safe driving. FMVSS 135 is the current standard and applies to 2000 and newer cars, and 2002 and newer light trucks. Compared to the earlier FMVSS 105 standard, FMVSS 135 requires roughly a 25% reduction in pedal effort for the same stopping distance.

FMVSS 135 says all vehicles under 10,000 lbs. gross vehicle weight (GVW), except motorcycles, must be capable of stopping within a distance of no more than 230 feet (70 meters) from 62 mph (100 km/h) with cold brakes (under 212 F or 100 C) and with no more pedal effort than 368 ft. lbs. (500 N).

In July 2005, these same requirements were extended to trucks and buses weighing more than 10,000 lbs. Formerly, only school buses had to meet the same stopping requirements as passenger cars and light trucks.

The FMVSS 135 standard also specifies a required stopping distance for vehicles should the power brakes fail (no power assist), or if one of the two hydraulic circuits fail. Under these conditions, the maximum stopping distance from 62 mph (100 km/h) is not to exceed 551 feet (168 meters) with a maximum pedal effort of no more than 368 ft. lbs. (500 N). FMVSS 135 also has a stopping requirement in the event of an anti-lock brake (ABS) system failure. The rules require the stopping distance not to exceed 279 feet (85 meters) with a maximum pedal effort of no more than 368 ft. lbs. (500 N).

There is also a hot performance stopping requirement for fade resistance. With the brakes hot, the maximum stopping distance for the second of two back-to-back panic stops is not to exceed 292 ft. (89 meters) with the same pedal effort as before (368 ft. lbs. or 500 N). The parking brakes are also covered by FMVSS 135. The rules specify conditions under which the parking brake must be able to hold the vehicle on both an uphill and downhill incline.
 
Coming down out of the mountains, 10 to 30 miles of downhill grade is not an uncommon situation.

If you charge near the top of a long grade, it might help to NOT charge to "full".

But, overheating mechanical brakes can be a real, significant issue.
 
garygid said:
Coming down out of the mountains, 10 to 30 miles of downhill grade is not an uncommon situation.

If you charge near the top of a long grade, it might help to NOT charge to "full".

But, overheating mechanical brakes can be a real, significant issue.
I'm not sure the Leaf will have an option for charging to less than full, and there's still the scenario where you need a full charge if you're driving east, but you're going to hit a long downhill if you drive west, and when you plug in the car at night you don't yet know where you'll be going the next day. And Nissan cannot put the burden on the driver, as most drivers will not be able to figure such planning into their charging routine. The car needs to be able to deal with it.

AndyH said:
daniel said:
All I'm asking is for Nissan to tell us if they've thought of this (I presume they have, but I'd like to hear them say it) and what particular method(s) they've implemented to deal with it.

Daniel - It's good that you care about having brakes that work. But let me start with a question: What do you drive now? Do you have a statement from the manufacturer that they sized the brakes correctly for the car? Actually, you do. If the car is certified to be on US roads, it must meet US safety standards - and that includes brake performance.
Actually, my daily driver now is a Zap Xebra, which has virtually no safety features at all, since it is classified and registered as a three-wheel motorcycle, and therefore does not fall under any safety regulations. It does have excellent brakes, and a hand brake, but no backup system comparable to compression braking. I made the decision that in order to be able to drive electric, three years ago, when there was virtually no other alternative, I was willing to accept the risks. Its principal safety feature is the fact that it only goes 35 mph. The Xebra at 35 mph is actually safer than a conventional car at 70 mph (the freeway speed limit here). And with a 40-mile maximum range (aftermarket batteries), there are no mountains I can reach.

I also own a Prius, but I only drive it when I have to go farther than about 35 miles, which means summer road trips to Canada for hiking in the mountains, and a few times a year when I go out of town.
 
Yes, you can charge less than full and many people do this who reside at the top of large hills.
 
I'm about 2 miles from the top of a hill (we are actually on a plateau) - and then I go down probably for a mile without any breaks. There are a lot of homes near the edge of the plateau who go down the hill as they come out of the home. So, this is definitely not a theoretical question in Seattle/San Francisco metro areas.
 
daniel said:
Actually, my daily driver now is a Zap Xebra, which has virtually no safety features at all, since it is classified and registered as a three-wheel motorcycle, and therefore does not fall under any safety regulations. It does have excellent brakes, and a hand brake, but no backup system comparable to compression braking. I made the decision that in order to be able to drive electric, three years ago, when there was virtually no other alternative, I was willing to accept the risks. Its principal safety feature is the fact that it only goes 35 mph. The Xebra at 35 mph is actually safer than a conventional car at 70 mph (the freeway speed limit here). And with a 40-mile maximum range (aftermarket batteries), there are no mountains I can reach.

I also own a Prius, but I only drive it when I have to go farther than about 35 miles, which means summer road trips to Canada for hiking in the mountains, and a few times a year when I go out of town.

That makes sense, Daniel. A car has three independent brake systems. The hydraulic system has two circuits - one controls the left front and right rear wheel; the second controls the right front and left rear wheel. Either circuit is required to safely stop the car even when the other is completely inoperative. In addition, the brake system must safely stop the car even if the power brake system dies, or the anti-lock system dies. In addition, the parking brake system is independent of the main brakes.

Engine compression and/or regen - though they might be available - are not part of the brake calculation. The friction brake system must be able to stop the car in both normal and emergency use. Period.

I hope that helps,
Andy
 
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